Control of fluid flows through a gas turbine engine is important to achieve efficiency and performance. Guide vanes are utilised in order to direct and present gas flows generated by the compressor and turbine stages of an engine. These vanes generally act between the stages of the engine. For example, a cascade of high-pressure turbine (HPT) nozzle guide vanes (NGVs) directs and guides the working gas flow from the combustor to the high pressure turbine.
FIG. 1 shows schematically a side view of an HPT NGV formation. The NGV formation has one or more aerofoil bodies 101 which span the working gas annulus of the engine, the direction of flow of the working gas being indicated. At radially inner and outer ends of the aerofoil bodies, the vane formation has respectively inner 102 and outer 103 platforms. The inner platforms of the cascade of vanes form an inner wall of the working gas annulus, while the outer platforms form an outer wall of the annulus. Conventionally HPT NGV formations are cooled internally with air which is fed from the high pressure compressor exit and passes through the inner and outer platforms to enter the ends of the aerofoil bodies, as shown in FIG. 1. The cooling air can eventually exit the aerofoil body through cooling holes formed in the aerofoil surfaces (where it can also provide surface film cooling) or air exits at the trailing edge of the aerofoil body.
The high-pressure cooling air is sealed from the annulus gas path by inner 104 and outer 105 chordal seals. Each chordal seal has a rail 106, 107 which projects from the respective platform 102, 103 and forms a respective contact face 108, 109 which sealingly contacts along a line with a corresponding contact face 110, 111 of inner and outer support rings of the engine. The chordal seal contact faces lie on prismatic surfaces (which may be cylindrical, as shown in FIG. 1, although other shaped surfaces are possible) while the support ring contact faces are essentially planar. Each chordal seal extends in a straight line between the circumferentially spaced side faces of the respective platform. The chordal seals 104, 105 act as mechanical restraints to react the axial gas loads, whilst allowing the NGV to articulate or pivot at the seals to compensate for differential axial thermal expansion effect without losing contact at the sealing faces.
In order to allow the NGV formations to articulate without losing contact at the contact faces 110, 111 the inner 112 and outer 113 contact lines of the chordal seals 104, 105 on the inner and outer support rings must be parallel, as shown in FIG. 2 by the projections of the contact lines 112, 113 onto the schematic plan view of the inner platform 102 of the NGV formation of FIG. 1, and the double line marks on each of these lines. However, due to engine design constraints, it is often not possible to have the chordal seals 104, 105 in the same axial position. An axial offset between the inner 112 and outer 113 contact lines then results.
In addition, each platform 102, 103 has an acute wedge angle between each side face 114 of the platform and the front 115 or rear 116 face of the platform. This angle is needed so that the intersection of the aerofoil bodies 101 with the inner and outer walls of the working gas annulus lies entirely within the platforms (i.e. the aerofoil bodies do not overhang the side faces of the platforms).
The net effect of these constraints is that there is typically a circumferential misalignment between the inner 112 and outer 113 contact lines, as shown in FIG. 2.
Desirably, the contact line of one chordal seal with its support ring ends at the same position that the contact line for the circumferentially adjacent chordal seal with that support ring begins. In this way, the contact lines with a given support ring can link up in a polygonal shape in which neighbouring chordal seals meet at “polygon corners”. This arrangement helps to reduce leakage around the ends of the chordal seal.
Typically, however, such linking up is only achieved for at most one of the inner and outer polygons, and “saw-tooth” leakage gaps result at the ends of the chordal seals of the other polygon as a result of the circumferential offset, as shown in FIG. 3. The leakage through this “saw-tooth” gap can be extremely penalising to turbine efficiency due the large pressure differential across the seal. The total area of all the gaps for a cascade of NGV formations can depend on a number of factors, but may be in excess of 50 mm2.